Yes, you can put an urban planter box directly on concrete without a saucer, but you should only do it if the planter is designed for it and you take a few precautions. A saucer is mostly there to catch runoff and protect the surface underneath. Without one, any excess water will drain straight onto the concrete. For outdoor concrete, that is not automatically a disaster, especially if the area slopes away and dries quickly. Indoors, however, it is usually a bad idea because water can pool, stain, or damage the floor.
The biggest risk is trapped moisture. If the planter sits flat against the concrete, air cannot move under it. The bottom stays damp, which can encourage mold, algae, and root rot. It can also cause white efflorescence, dark water rings, or discoloration on the concrete. Metal planters may leave rust stains; wood can leach tannins; terracotta can leave mineral marks. Over time, repeated wetting and drying can also affect a concrete sealer or paint.
If you want to skip the saucer, raise the planter slightly. Use pot feet, bricks, cedar blocks, or a slatted riser so there is an air gap. This lets water drain and lets the bottom dry. Make sure the planter has working drainage holes and never let it sit in a puddle. A waterproof mat or tray underneath can still protect the concrete without looking like a traditional saucer. If the concrete is sealed, choose a mat that will not trap condensation.
Watering matters just as much. Water deeply but less often, and stop before water pours out the bottom. In rainy seasons, check the planter after storms. If runoff is heavy, add a shallow drip tray or move the box under cover. On a balcony, remember that water can drip below, so a saucer or liner may be the neighbor-friendly choice.
In short, direct contact without a saucer can work outdoors if the planter has feet, good drainage, and a protective barrier. But if you want to protect the concrete and your plants, a saucer, tray, or hidden liner is still the safer option. If the planter is indoors or on finished concrete, do not skip it.